1. Technical Field
The invention relates to upper internal equipments of water-cooled and moderate nuclear reactors of the type having a vessel provided with lateral water outlet nozzles and closed by a lid, said equipment having an upper core plate and a plate supporting the cluster guides which connect the two plates together and have a portion projecting above the support plate.
The invention is particularly, although not exclusively, suitable for use in so-called "advanced" pressurized water nuclear reactors, of the under-moderated and/or spectral shift type, whose fuel assemblies are often distributed in the core with closer spacing than in the thermal neutron reactors at present in service. The reduction in spacing results in a closer arrangement of the cluster guides which reduces the flow cross-sectional area offered to the coolant leaving the core upwardly and whose flow lines are curved towards the nozzles through the lattice of cluster guides.
The flow cross-section left free is even smaller in reactors with spectral shift, which comprise not only guides for the absorbent control clusters but also guides for the spectrum variation clusters which modify the volume of water in the core and possibly (in the case of fuel rod clusters containing fertile material) further harden the spectrum by selective absorption of slow neutrons.
2. Prior Art
In present day thermal pressurized water reactors, such as those described in European Patent No. 079,828 (Framatome) and French Patent No. 2,399,715 (Westinghouse), the pressurized water leaving the core is directed to the outlet nozzles through the upper internal equipment between the cluster guides and, if provided, spacers which connect the plates together. In the prior art arrangement shown by way of example in FIG. 1, core 10 has the upper internal equipment mounted thereover. This equipment includes an upper core plate 12 which is rigidly fixed to a plate 14, supporting guides 26, by evenly distributed spacers 16. The support plate 14 is a solid part having a bottom wall to which the spacers 16 are fixed, connected by a sleeve 18 to a flange 20 supported by the vessel 22 of the reactor, closed by a cover 24.
The cluster guides 26 are disposed at locations between the spacers, in alignment with those of the fuel assemblies which are to receive clusters. As shown in FIG. 1, the cluster guides 26 are in two parts. The lower part, placed in the internal equipment, is provided with a connection flange bearing on the upper surface of the bottom wall of the support plate 14. The lower end portion of the lower part is engaged in a guide hole formed in the upper core plate 12. The upper part of each cluster guide is fixed by flanges and bolts to the respective lower part.
As shown, water leaves the core through openings formed in the upper core plate 12 between the cluster guides and/or at the foot of spacers 16 and through windows 28 formed in the lower zone of the cluster guides. The cluster guides each carry vertically distributed horizontal guide plates 30 having a cut out corresponding to the distribution of the rods of the cluster.
The bottom wall of the support plate 14 is at a level appreciably higher than that of the outlet nozzles 32. The lengths between bearing points, L1 for the cluster guides and L2 for the spacers, are important and impart to the respective members a flexibility which, if the cross-sectional flow area left for the coolant is appreciably less than that found in present day reactors and if therefore the flow speed is increased, may result in unacceptable vibrations.